The present invention relates to a liquid crystal display device applying Twist OCB mode, and, more particularly, to a liquid crystal display device having stairs type driving signal which can eliminate over-transmittance due to voltage application and have rapid response time by successively applying data signals having different size when a gate pulse is applied.
Generally, liquid crystal display devices have a structure that liquid crystal molecules are aligned between a pair of transparent substrates. In the structure, screen display is performed by transmitting or cutting off light for electrically controlling the alignment state of liquid crystal molecules. The liquid crystal display device has been used as an indicator of electronic calculators and digital watches and at the same time, rapidly extended its application to a screen of laptop computer, television receiver and word processor.
The conventional liquid crystal display has generally employed TN mode which has improved optical properties, clear B/W display and rapid response time. As shown in FIG. 1, the TN mode liquid crystal display device comprises an upper and a lower substrates 1, 5, a pixel electrode 2 arranged on the inner side of lower substrate 1, a common electrode 6 arranged on the inner side of upper substrate 5, alignment layers (not shown) respectively arranged on the opposite surfaces of upper and lower substrates, polarizing plates 4,7 on the outer sides of upper and lower substrates 1,5 and a liquid crystal layer 8 interposed between upper and lower substrates 1,5, including a plurality of liquid crystal molecules.
The alignment layer (not shown) is a horizontal alignment layer wherein a rubbing axis 3b thereof is crossed by 90xc2x0 and upper and lower polarizing plates and polarizing axes 4a, 7a are attached in a crossed direction. That is, the rubbing axis 2a of lower alignment layer and the polarizing axis 4a of lower polarizing plate are attached in the same direction, and the rubbing axis 3b of upper alignment layer and the polarizing axis 7a of upper polarizing plate are attached in the same direction. And, the pixel electrode 2 and the common electrode 6 are formed in a plate shape.
As shown in FIG. 1, liquid crystal molecules 8 are levorotatorily twisted by 90xc2x0 under the influence of upper and lower alignment layers and chiral dopant before the electric field is applied to the region between the pixel electrode 2 and the common electrode 6. Therefore, after passing through the lower polarizing plate 4, light can pass through levorotatorily twisted liquid crystal molecules 8a, and then the upper polarizing plate 7. As a result, the screen becomes white.
Although it is not shown in the drawings, when the electric filed is applied to the region between the pixel electrode 2 and the common electrode 6, the liquid crystal molecules 8a are arranged to be parallel with the electric field (perpendicular to the substrate) formed between driving electrodes. Therefore, after passing through the lower polarizing plate 4, light cannot pass through the crossed upper polarizing plate since the major axis of liquid crystal molecules 8a is perpendicular to the surface of substrate. As a result, the screen becomes dark.
However, the TN liquid crystal display device has different refractive anisotropies according to the direction since the interposed liquid crystal molecules have a shape of bar. Thus, transmittance is drastically diversified according to the viewing angle and therefore, the liquid crystal display has disadvantages that it is difficult to apply to large scale display and the response time is too slow to realize moving pictures. As a result, it has difficulty in being applied to large scale TV due to its narrow viewing angle and slow response time.
In order to solve the problems, various liquid crystal modes have been proposed. However, they have not completely satisfied wide viewing angle and at the same time, high speed response time. Recently, OCB (Optically Compensated Bend) mode has been proposed to have a wide viewing angle by a phase compensation film and at the same time, a high speed response time below 10 ms. The OCB is a mode using bend of liquid crystals generated when the upper and lower substrates are rubbed to be parallel with each other and a predetermined voltage is applied.
The OCB mode liquid crystal display (Reference: SID 93 Digest P277, xe2x80x9cWide-Viewing-Angle Display Mode for the Active-Matrix LCD Using Bend-Alignment Liquid Crystal Cell, Y. Yamaguchi, T. Miyashita, T. Uchida) can compensate refractive anisotropy of liquid crystal molecules without several times of rubbing processes, thereby maintaining regular viewing angle in any direction of screen.
FIGS. 2A to 2C are drawings showing a conventional OCB mode liquid crystal display device.
Referring to FIG. 2A, a lower substrate 10 and an upper substrate 15 are arranged opposite to each other with a predetermined distance. A liquid crystal layer 18 is interposed between the lower substrate 10 and the upper substrate 15. The liquid crystal layer includes a plurality of liquid crystal molecules 18a, made of materials having positive dielectric anisotropy. And, driving electrodes 11,16 are arranged on the inner sides of lower and the upper substrates 10,15 to drive liquid crystal molecules, wherein a first alignment layer 12 is disposed on the inner side of lower substrate 10, that is, between the lower substrate 10 and the liquid crystal layer 19 and a second alignment layer 17 is disposed on the inner side of upper substrate 15, that is, between the upper substrate 15 and the liquid crystal layer 19. The first and the second alignment layers 12,17 are horizontal alignment layers having a pretilt angle of below 10xc2x0, rubbed to the direction parallel with each other. Furthermore, polarizing plates 19a,19b are attached on the outer sides of lower and upper substrates 10,15, having a predetermined of polarizing axes. It is desirable that the polarizing axes of the polarizing plates 19a,19b are cross-arranged with each other.
As shown in FIG. 2A, when the voltage is not applied to the OCB mode liquid crystal display device, liquid crystal molecules 18a are arranged in a shape of splay under the influence of first and second alignment layers 12,17.
As shown in FIG. 2B, when the voltage is applied between the driving electrodes to the critical voltage Vs, that is, as much as liquid crystal molecules 18a in the middle layer of the liquid crystal layer 18 are affected by electric field, the liquid crystal molecules 19a in the middle layer are twisted by the effect of electric field E1, so that the electric field and the major axis thereof are parallel with each other. However, the liquid crystal molecules 19a arranged on upper and lower parts are affected by alignment layers 12,17 than by electric field, thereby maintaining the initial alignment. Here, it is possible to control dxcex94n of liquid crystals to make white state.
Thereafter, as shown in FIG. 2C, when the voltage greater than the critical voltage Vs is applied between driving electrodes, liquid crystal molecules are affected by electric field E2 in the middle layer and the vicinities thereof. Therefore, they are twisted so that the electric field and the major axis thereof are parallel with each other, thereby screen becomes dark. The liquid crystal molecules 19a adjacent to the surface of substrates 10,15 are affected by alignment layers 12,17 than by electric field, thereby maintaining the initial alignment.
In the OCB mode liquid crystal display device, liquid crystal molecules 19a are arranged symmetrically with respect to the middle layer when the electric field is formed. Therefore, it is possible to accomplish phase compensation when light passes through the upper substrate 15 from the lower substrate 10. And, when the electric field is not formed, backflow is not generated, having a rapid response time.
According to the conventional OCB mode liquid crystal display, when the electric field is not formed, liquid crystal molecules are arranged horizontally with substrate surface and spacers are distributed to maintain cell gap. Here, liquid crystal molecules around spacers are arranged along the surface of spacer, not maintaining horizontal alignment. As a result, liquid crystal molecules are arranged unstably in the region where spacers are distributed.
And, driving electrodes 11,16 are formed in a pattern type, having a predetermined topology. Therefore, on the upper part of pixel electrode 11 or counter electrode, the major axis of liquid crystal molecules forms parallel with the surface of horizontal alignment layer 12 by the influence of the horizontal alignment layer 12. However, in the topology of pixel electrode 11, the major axis of liquid crystal molecules forms parallel with the surface of topology, thereby generating reverse tilt.
As described above, the alignment of liquid crystal molecules is partially unstable and reverse tilt is generated in the topology, thereby deteriorating screen qualities.
In order to solve the problems, COCB (Chiral Optical Compensated Bend) mode has been proposed, wherein chiral dopants are added to the liquid crystal in order to accomplish stable twist in the range of a predetermined d/p (cell gap/pitch) before voltage application or at one initial voltage application, thereby overcoming problems of normal OCB mode. FIG. 3 shows the range of applied voltage according to din in the COCB mode.
However, as shown in FIG. 3, when voltage is applied to obtain desirable transmittance, the transmittance is increased not promptly but through another transmittance. Therefore, in the conventional COCB mode liquid crystal display device, when small voltage is applied, the response time is increased.
Therefore, an object of the present invention is to provide a liquid crystal display device having stairs type driving signal capable of preventing over twist of liquid crystals and improving response time properties by applying stairs type pulse signal when a gate signal is applied to drive TFT.
In order to accomplish the above object, the present invention comprises:
an upper and a lower substrates arranged opposite to each other with a predetermined distance;
a liquid crystal layer interposed between the upper and lower substrates, comprising liquid crystals having refractive anisotropy;
a first and a second driving electrodes arranged on the inner sides of upper and lower substrates to drive the liquid crystal;
a first alignment layer interposed between the lower substrate and the liquid crystal layer, rubbed to a first direction;
a second alignment layer interposed between the upper substrate and the liquid crystal layer, rubbed to the direction parallel with the first alignment layer;
a first polarizing plate arranged on the outer side of lower substrate, forming an angle of 45xc2x0 with the first direction; and
a second polarizing plate arranged on the outer side of upper substrate, transmission axis thereof being perpendicular to the first polarizing plate and forming an angle of 45xc2x0 with the first direction, and
when a gate signal is applied to drive thin film transistors in each pixel, data signals having two or more stairs type pulse signals are successively applied.
The amount of dopants is controlled to maintain cell gap (d)/pitch of liquid crystal (p) at 0.1xcx9c0.8, desirably 0.26xcx9c0.36 according to the cell gap (d). Therefore, although they are arranged in splay or in a mixture of splay and twist before voltage application, it is possible to maintain 180xc2x0 twisted state after twisted by applying a predetermined voltage.
The d/p is controlled at 0.36xcx9c0.8 to maintain stable twist before voltage application.
The refractive anisotropy of liquid crystal is approximately xcex94n=0.06xcx9c0.18, the cell gap (d) is 3xcx9c20 xcexcm and dxcex94n is 0.18xcx9c3.6 xcexcm.
In the liquid crystals, dielectric anisotropy is approximately xcex94xcex5=2xcx9c20 to lower driving voltage.
The liquid crystals have a phase retardation of approximately dxcex94n=0.02xcx9c1.5 xcexcm at the front thereof and refractive anisotropy perpendicular or horizontal to rubbing axis of the first direction, thereby becoming dark state at a desired voltage.
The liquid crystal has a pretilt angle of 0xcx9c40xc2x0.
The stairs type pulse signal has two or more voltage levels.